DNA polymerase inhibitors are commercially available to inhibit the replication of DNA viruses (cidofovir [Visitide®], valacyclovir hydrochloride [Valtrex®], famciclovir [Famvir®, acyclovir [Zovirax]). They also have an affinity for mammalian DNA, but to a lesser extent. Each of these currently available drugs has various degrees of affinity for DNA polymerase inhibition. Acyclovir is an effective drug for the treatment of Herpes Simplex Virus (HSV) and Varicella-Zoster Virus (VZV) infections, which after phosphorylation to the triphosphate inhibits viral DNA polymerase. Acyclovir has low oral bioavailability, therefore prodrugs have been developed, such as the L-valyl ester, valaciclovir, that treats shingles. Ganciclovir is used against Cyto-Megalo-Virus CMV, and famciclovir, a lipophilic prodrug of penciclovir, is marketed for shingles. The acyclic nucleoside phosphonates are active against thymidine kinase-resistant viral strains. Oligonucleotides incorporating acyclic nucleosides at the 3′- and 5′-ends, or constituted of amino acyclic nucleosides, are resistant to cleavage by nucleases and may be useful in antisense and/or antigene therapy. Some acyclic nucleosides are potent inhibitors of purine and pyrimidine nucleoside phosphorylase.
Nucleoside phosphonates (ANPs) bring a new dimension to the therapy of viral infections, as they offer a broader spectrum of activity, a longer duration of antiviral action and a lower risk of resistance development compared with available treatments. The key factor underlying all these unique features is the presence of the phosphonate group, which allows ANPs to interfere with the normal pathway of nucleic acid biosynthesis, and, in particular, viral nucleic acid biosynthesis. Three ANPs (cidofovir, adefovir and tenofovir) have been marketed worldwide. They are active against virtually all key DNA viruses and retroviruses.
ANPs behave as analogues of 2′,3′-dideoxynucleotides. In contrast to the ‘classical’ acyclic nucleoside analogues, such as acyclovir, ganciclovir or penciclovir, or dideoxynucleoside analogues, such as zidovudine (AZT) or lamivudine (3TC), they do not require the initial phosphorylation needed for the activation of modified nucleosides, which is catalysed by nucleoside kinase. In those cells in which the nucleoside kinase is less active or completely missing, nucleoside analogues are inactive, whereas ANPs are converted by nucleotide kinase (GMP kinase or AMP kinase) to the monophosphate (an analogue of diphosphate) and further by nucleoside diphosphate (NDP) kinase to the triphosphate analogue. These di- and triphosphate analogues are inhibitors/substrates of the respective enzymes. However, the true active species are the triphosphate (ANPpp) analogues that target DNA polymerase—viral and/or cellular. The inhibition differs with the character of the base, with the most potent inhibition occurring with guanine derivatives. This was observed with DNA polymerase from different sources—viral and mammalian, as well as with enzymes from transformed cells or cellular parasites. However, ANPpp analogues are also substrates of DNA polymerases: consequently, the elongation of the DNA chain catalysed by diverse DNA polymerases and Reverse transcriptases (RTs) comes to a standstill after the incorporation of 9-[2-(phosphonomethoxy) ethyl] (PME) or (R)- or (S)-9-[2-(phosphonomethoxy)propyl] (PMP) compounds—a situation that is typical of chain-terminators (for example, AZT and 3TC).
The antiviral activity of ANPs is the result of the higher affinity of the diphosphorylated ANP metabolite for viral DNA polymerases (that is, HSV-1 DNA polymerase, CMV DNA polymerase and HIV-1 RT) than for the cellular DNA polymerases. As conversion of the ANPs to the monophosphate stage does not depend on the virus-induced thymidine kinase (as opposed to, for example, the antiherpes drug acyclovir), they should exert activity against a broad range of DNA viruses. Indeed, all DNA viruses and retroviruses have been found to be susceptible to cidofovir, adefovir and/or tenofovir. Cidofovir ((S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine (HPMPC)) is phosphorylated by pyrimidine nucleoside monophosphate kinase to cidofovir monophosphate (HPMPCp), which is then further phosphorylated by nucleoside diphosphate kinase, pyruvate kinase or creatine kinase to cidofovir diphosphate (HPMPCpp). These two phosphorylation steps can occur in both uninfected and virus-infected cells. For adefovir (PMEA), and presumably also for tenofovir ((R)-9-[2-(phosphono-methoxy)propyl]adenine; PMPA), phosphorylation to the diphosphate form (PMEApp and PMPApp, respectively) requires two steps, both of which depend on the AMP (dAMP) kinase.
Cidofovir (a nucleoside analogue of deoxycytidine) is an intravenous anti-viral drug most active against DNA viruses (such as Human Papilloma, pox, and herpes simplex). Cidofovir has proved to be effective in the treatment of herpes-, papilloma-, polyoma-, adeno- and pox-virus infections. It has been formally approved for intravenous use in the treatment of cytomegalovirus retinitis in AIDS patients.